1. Field of the Invention
The present invention relates to an image forming system and an image forming method that can conduct a charging control for the image carrier.
2. Description of the Related Art
In many conventional image forming systems like electrophotographic devices and electrostatic recording devices, the non-contact type corona charging method is usually employed to charge the surface of the image carrier made of, for example, photosensitive material and dielectric material. According to this charging method, corona produced by loading high voltage onto a thin corona discharging wire is provided to the surface of the image carrier for charging.
In recent years, however, because of advantages of low ozone-emission and low cost, such a contact type charging method has become popular that charges the image carrier by applying voltage to a roller or blade type charging unit and then containing it to the surface of the image carrier.
Particularly, the roller type charging unit shows stable charging performance over a long time. Although the charging unit can work with DC voltage alone, it is charged uniformly if AC voltage is used to repeat plus and minus discharging alternatively. For example, a DC voltage(DC offset bias) is overlapped with an AC voltage that has an inter-peak voltage higher than the charging unit's discharge threshold(discharge initiating DC voltage). It is known that such voltage vibrations make charges on the charging unit uniform. The waveforms of voltage vibration may be sinusoidal, rectangular, triangular or pulse-like. Voltage vibrations include the rectangular voltage waves generated by turning on/off DC voltage at regular intervals and overlap of AC voltage and DC voltage where the level of DC voltage is periodically modulated.
The contact type charging method that charges a charging unit by applying such voltage vibrations thereto is referred to as the AC charging method here. On the other hand, the contact type charging method using only DC voltage for charging is referred to as the DC charging method. Compared with the DC charging method, the AC charging method provides more charges to the image carrier and thus the degradation of the image carrier, for example, wear is accelerated. In addition, it occasionally poses problems such as image escape due to charge products under high-temperature, high-humidity conditions.
To solve such problems, the applied voltage and the alternative (plus and minus) discharge must be minimized. In reality, however, the amount of discharge is not fixed only by the applied voltage but changes with various factors such as the film thickness of the photosensitive layer or dielectric layer of the image carrier and conditions of the charging unit and ambient air. Under low temperature, low humidity (L/L) conditions, the charging material is dried and its resistivity rises, making discharge harder to occur. Thus the inter-peak voltage must be higher than a certain level for uniform charging. Meanwhile, during charging under high temperature, high humidity (H/H) conditions, the charging material gets damp and its resistivity decreases. As a result, under H/H conditions, the charging unit produces more charges than necessary even at the minimum voltage applied for uniform charging under L/L conditions. Then the amount of charge increases and such problems arise that image quality is impaired, fused toner adheres and the image carrier lasts shorter because of surface wear.
In addition to variations in the surrounding conditions, fluctuations in the manufactured charging unit, resistivity variations due to stains, changes in the capacitance of the image carrier over time and performance fluctuations in the high voltage supply device lead to fluctuations in the amount of charge. As a technique to reduce fluctuations in the amount of charge, Japanese Patent Application Laid-open No. 2001-201921 has disclosed a discharge current control method. In this method, the AC voltage applied to the charging unit is variable in the image forming system. Specifically, the individual AC currents at two or more voltages, some of which are lower than the discharge threshold and others higher than that, are detected, and such AC voltage is calculated that provides the best amount of charge based on the detected AC currents, in order to determine the AC voltage to be loaded to the charging unit.
The above charging current control method, however, has the following problem. When there are detection errors during current detection, it is difficult to precisely measure AC current at a voltage lower than the discharge threshold voltage and AC current at a voltage higher than the discharge threshold voltage, and therefore the charge voltage level providing the best charging is not calculated.
Current detection accuracy can be raised by narrowing the range of current detection.
However, it is difficult to narrow the detection range because of a large gap between the current at a voltage lower than the discharge threshold voltage and the current at a voltage higher than the discharge threshold voltage, and eventually the detection accuracy decreases.
In this invention, current detection accuracy is improved by switching the detection ranges of AC current for charging between voltages lower than the discharge threshold and voltages higher than the threshold and by detecting current in the individual optimized (narrow) detection ranges. Then fluctuations in the detected current become small. As a result, problems such as degradation of the image carrier caused by excessive charging produced by performance fluctuations of the charging unit and ambient conditions can be prevented. An object of the invention is therefore to provide an image forming system and an image forming method that can maintain high image quality over a long time by preventing such problems.